Vehicles, and particularly all-terrain vehicles (ATV's) are intended for use on a variety of different surfaces, including dry pavement, dirt, grass, slippery surfaces such as ice, and gravel. Typically, such vehicles are driven by their back wheels, but allowance is made for providing torque also to the front wheels when the back wheels slip, as, for example, when a vehicle becomes mired in mud. It is not uncommon for an ATV to have a rear differential to enable it to be steered easily in a tight circle without dragging a rear wheel on the ground and doing damage to the surface upon which the vehicle is being driven, i.e., turf at a golf course. On the other hand, it is often important to be able to lock the rear differential so that both rear wheels, rather than simply the slipping or mired wheel, is driven. Differential gearing also can be employed at the front of a vehicle, as when the front wheel drive is continuously engaged. Here, either the front differential, the rear differential, or both, may be “locked up” so that torque is delivered by the engine to each wheel. Front wheel drive vehicles in which both front wheels are continuously driven, as when a front differential is locked, commonly are difficult to steer and accordingly are unpleasant to drive.
Double acting overrunning clutches have been employed in certain operations to allow a driven member to rotate faster than a drive member; see, for example, U.S. Pat. No. 3,194,369. Roller clutches have been used in transfer cases of certain four wheel drive vehicles to engage the front axle when the rear wheels begin to spin; see U.S. Pat. No. 3,295,625. U.S. Pat. No. 5,036,939 describes a vehicle having a pair of steerable wheels, each steerable wheel including a hub having a double acting overrunning clutch. Here, the front axle is preferably geared so that the front wheels are driven by the ground at a rotational speed approximately 10-30% faster than the front axle such that when the rotational speed of the front axle catches up to the ground-driven rotational speed of the front wheels (as when the rear wheels are spinning), the clutch will transmit torque from the front axles to the front wheels.
It would be desirable to provide a four wheel vehicle such as an ATV that would automatically transfer torque to one or both front wheels in response to slippage of either or both of the rear wheels, and would accomplish this purpose in a manner that would continue to provide torque to the front wheels only when needed.